Systems and methods for charged particle beam modulation

ABSTRACT

Systems and methods for conducting charged particle beam modulation are disclosed. According to certain embodiments, a charged particle beam apparatus generates a plurality of charged particle beams. A modulator may be configured to receive the plurality of charged particle beams and generate a plurality of modulated charged particle beams. A detector may be configured to receive the plurality of modulated charged particle beams.

TECHNICAL FIELD

The present disclosure generally relates to the field of chargedparticle beam systems, and more particularly, to systems and methods formodulating a charged particle beam.

BACKGROUND

Charged particle beam systems are employed in various fields, such aselectron microscopy, lithography, particle acceleration, and others. Asone example, electron microscopes are useful tools for observing thesurface topography and composition of a sample. In an electron beam toolused for microscopy, electrons are directed to a sample and may interactwith the sample in various ways. For example, when an electron beamimpinges on a sample, secondary electrons, backscattered electrons,auger electrons, x-rays, visible light, etc. may be scattered from thesample and directed to a detector. Scattered particles may form one ormore beams incident on the detector.

To increase throughput, a multi-beam imaging (MBI) system may split aprimary electron beam into a plurality of beamlets for scanning multipleseparate areas of a sample simultaneously. After impinging on thesample, a plurality of beamlets of scattered or secondary electrons maybe directed onto a detector. Typically, a detector for use in an MBIsystem may be provided with a plurality of sensing elements, forexample, in the form of a pixelated array. Due to the effects ofaberration and dispersion, multiple beam spots may overlap on thedetector surface, leading to crosstalk. Furthermore, the beam spots maydrift and change the locations at which detector sensing elements areactivated for sensing the beam spot. Additional optical elements may berequired to track the multiple beam spots and correct the projection ofthe beamlets, contributing to complexity and adding difficulties toscaling up detector systems. Additionally, when a fine detector arraycomprising a large number of sensing elements is provided, a switchingmatrix should also be provided to connect individual sensing elementsassociated with the same beam spot, introducing further complexity.

SUMMARY

Embodiments of the present disclosure provide systems and methods forcharged particle beam modulation. In some embodiments, a chargedparticle beam system is provided. The charged particle beam system mayinclude a charged particle beam apparatus configured to generate acharged particle beam.

In some embodiments, a charged particle source of a charged particlebeam system is configured to generate a plurality of charged particlebeams. A modulator may be provided that is configured to receive theplurality of charged particle beams and generate a plurality ofmodulated charged particle beams. Furthermore, a detector may beprovided that is configured to receive the plurality of modulatedcharged particle beams. The detector may include a detector array of oneor more sensing elements.

According to some embodiments, an arrangement can be achieved that canenhance and simplify a detection branch of a charged particle beamsystem.

Additional objects and advantages of the disclosed embodiments will beset forth in part in the following description, and in part will beapparent from the description, or may be learned by practice of theembodiments. Objects and advantages of the disclosed embodiments may berealized and attained by the elements and combinations set forth in theclaims. However, exemplary embodiments of the present disclosure are notnecessarily required to achieve such exemplary objects and advantages,and some embodiments may not achieve any of the stated objects andadvantages.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosed embodiments, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary electron beaminspection (EBI) system, consistent with embodiments of the presentdisclosure.

FIG. 2A is a schematic diagram illustrating an exemplary multi-beamelectron beam tool, consistent with embodiments of the presentdisclosure that can be a part of the exemplary electron beam inspectionsystem of FIG. 1.

FIG. 2B is a diagram illustrating a source conversion unit, consistentwith embodiments of the present disclosure.

FIG. 3 is a schematic diagram illustrating an exemplary single-beamelectron beam tool, consistent with embodiments of the presentdisclosure that can be a part of the exemplary element beam inspectionsystem of FIG. 1.

FIG. 4 is a schematic diagram illustrating exemplary transmission typeelectron beam tool that can be part of the exemplary electron beaminspection system of FIG. 1, consistent with embodiments of the presentdisclosure.

FIG. 5 is a diagram illustrating an exemplary comparative detectionscheme, consistent with embodiments of the present disclosure.

FIG. 6 is a diagram illustrating an exemplary surface of a detector,consistent with embodiments of the present disclosure.

FIG. 7 is a diagram illustrating an exemplary surface of a detector,consistent with embodiments of the present disclosure.

FIG. 8 is a diagram illustrating an exemplary detection scheme employingmodulation, consistent with embodiments of the present disclosure.

FIG. 9 is a diagram illustrating exemplary waveforms representing beammodulation, consistent with embodiments of the present disclosure.

FIGS. 10A and 10B illustrate exemplary deflectors, consistent withembodiments of the present disclosure.

FIG. 11 is a schematic diagram illustrating an exemplary electron beamtool, consistent with embodiments of the present disclosure that can bea part of the exemplary element beam inspection system of FIG. 1.

FIG. 12 is a schematic diagram illustrating an exemplary electron beamtool, consistent with embodiments of the present disclosure that can bea part of the exemplary element beam inspection system of FIG. 1.

FIG. 13 is a diagram illustrating an exemplary surface of a detector,consistent with embodiments of the present disclosure.

FIG. 14 is a diagram illustrating an exemplary modulation anddemodulation scheme, consistent with embodiments of the presentdisclosure.

FIG. 15 is a diagram illustrating exemplary waveforms representing beammodulation, consistent with embodiments of the present disclosure.

FIG. 16 is a diagram illustrating exemplary waveforms representing beammodulation, consistent with embodiments of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses, systems, and methods consistent with aspectsrelated to the subject matter as recited in the appended claims. Forexample, although some embodiments are described in the context ofutilizing electron beams, the disclosure is not so limited. Other typesof charged particle beams can be similarly applied.

Embodiments of the present disclosure may provide a charged particlesystem that may be useful for charged particle imaging. The chargedparticle system may be applicable as a scanning electron microscope(SEM) tool for imaging and inspecting a sample.

In some exemplary embodiments, modulation is applied to a chargedparticle beam. A modulator may modulate a plurality of beamlets of anMBI system. In some embodiments, a modulator may modulate a plurality ofbeams generated from a single primary electron beam. As used herein, thephrases “a plurality of beams” or “a plurality of charged particlebeams” may refer to a plurality of beamlets or one or more other beams.

Modulation is used in data processing and communications applications tomultiplex a signal of digital data for transmitting over one or moretransmission channels. For example, code-division multiple access (CDMA)is a type of multiplexing that allows various signals to occupy a singletransmission channel. Generally, CDMA techniques involve spread spectrummultiple-access methods where bandwidth is spread uniformly for the sametransmitted power. A spreading code may be provided that is apseudo-random code (PN code) used for differentiating signals associatedwith different users. Individual signals can be extracted from the maindata signal based on a correlation of received signals with locallygenerated code.

In an analogous manner, modulation may be applied to a charged particlebeam to divide the beam into a plurality of modulated beam portions.

Modulation involves a tradeoff between the number of individual signalsand bandwidth in the frequency domain. For example, two signals can betransmitted at 50 MHz, or one signal can be transmitted at 100 MHz basedon the same information content. Thus, in a communications setting, avery high density of users can be accommodated on a single signalcarrier line, but bandwidth allocated to each user dwindles.

In a similar manner, an MBI system may involve a large number ofbeamlets that may each represent a unique subset of topographyinformation. An MBI system may divide a primary charged particle beaminto a plurality of beamlets. A large number of beamlets may interactwith a sample and be directed onto a detector. At the detector, thedeflected beamlets may simultaneously irradiate the detector surface atdifferent locations. An area detector can be provided with a pluralityof sensing elements that receives and transmits signals assigned to eachbeamlet. Furthermore, because the shape and/or position of a beamlet maynot necessarily exactly correspond to a sensing element, the detectormay be broken up into a fine array including a large number of smallsensing elements. For example, a single beamlet may be sensed bymultiple sensing elements, each of which may not fully encompass thebeamlet. Manipulation of the connections between sensing elements canenable the outputs of corresponding beamlets to be collected together.That is, outputs of a plurality of sensing elements may be summed toobtain a corresponding output of one beamlet.

Thus, a detector with a large number of sensing elements may be requiredto receive energy from all of the beamlets while being able to adjustfor deviations in the shape and location of the beamlets. A large numberof sensing elements may be necessary in order to ensure spatiallyseparation among the beamlets and prevent crosstalk. For example, anarea detector array of 21×21 sensing elements may comprise 21×21channels to transmit signals. Each channel may have a sampling rate of100 MHz and bandwidth of 33 MHz.

In some embodiments, modulation can be applied to the beamlets to reducethe number of signals. According to some embodiments, modulation may beuseful in enhancing and/or simplifying detector systems.

Reference is now made to FIG. 1, which illustrates an exemplary electronbeam inspection (EBI) system 100 consistent with embodiments of thepresent disclosure. As shown in FIG. 1, EBI system 100 includes a mainchamber 101 a load/lock chamber 102, an electron beam tool 104, and anequipment front end module (EFEM) 106. Electron beam tool 104 is locatedwithin main chamber 101. EFEM 106 includes a first loading port 106 aand a second loading port 106 b. EFEM 106 may include additional loadingport(s). First loading port 106 a and second loading port 106 b receivewafer front opening unified pods (FOUPs) that contain wafers (e.g.,semiconductor wafers or wafers made of other material(s)) or samples tobe inspected (wafers and samples may be collectively referred to as“wafers” hereafter).

One or more robotic arms (not shown) in EFEM 106 may transport thewafers to load/lock chamber 102. Load/lock chamber 102 is connected to aload/lock vacuum pump system (not shown) which removes gas molecules inload/lock chamber 102 to reach a first pressure below the atmosphericpressure. After reaching the first pressure, one or more robotic arms(not shown) may transport the wafer from load/lock chamber 102 to mainchamber 101. Main chamber 101 is connected to a main chamber vacuum pumpsystem (not shown) which removes gas molecules in main chamber 101 toreach a second pressure below the first pressure. After reaching thesecond pressure, the wafer is subject to inspection by electron beamtool 104. Electron beam tool 104 may be a single-beam system or amulti-beam system. A controller 109 is electronically connected toelectron beam tool 104. Controller 109 may be a computer configured toexecute various controls of EBI system 100. While controller 109 isshown in FIG. 1 as being outside of the structure that includes mainchamber 101, load/lock chamber 102, and EFEM 106, it is appreciated thatcontroller 109 can part of the structure.

FIG. 2A illustrates an electron beam tool 104 that may be configured foruse in EBI system 100. Electron beam tool 104 may be a multi-beamapparatus, as shown in FIG. 2A, or a single beam apparatus, as shown inFIG. 3.

FIG. 2A illustrates an electron beam tool 104 (also referred to hereinas apparatus 104) that may be configured for use in a multi-beam imaging(MBI) system. Electron beam tool 104 comprises an electron source 202, agun aperture 204, a condenser lens 206, a primary electron beam 210emitted from electron source 202, a source conversion unit 212, aplurality of beamlets 214, 216, and 218 of primary electron beam 210, aprimary projection optical system 220, a wafer stage (not shown in FIG.2A), multiple secondary electron beams 236, 238, and 240, a secondaryoptical system 242, and an electron detection device 244. Primaryprojection optical system 220 can comprise a beam separator 222,deflection scanning unit 226, and objective lens 228. Electron detectiondevice 244 can comprise detection sub-regions 246, 248, and 250.

Electron source 202, gun aperture 204, condenser lens 206, sourceconversion unit 212, beam separator 222, deflection scanning unit 226,and objective lens 228 can be aligned with a primary optical axis 260 ofapparatus 104. Secondary optical system 242 and electron detectiondevice 244 can be aligned with a secondary optical axis 252 of apparatus104.

Electron source 202 can comprise a cathode, an extractor or an anode,wherein primary electrons can be emitted from the cathode and extractedor accelerated to form a primary electron beam 210 with a crossover(virtual or real) 208. Primary electron beam 210 can be visualized asbeing emitted from crossover 208. Gun aperture 204 can block offperipheral electrons of primary electron beam 210 to reduce Coulombeffect. The Coulomb effect may cause an increase in size of probe spots.

Source conversion unit 212 can comprise an array of image-formingelements and an array of beam-limit apertures. The array ofimage-forming elements can comprise an array of micro-deflectors ormicro-lenses. The array of image-forming elements can form a pluralityof parallel images (virtual or real) of crossover 208 with a pluralityof beamlets 214, 216, and 218 of primary electron beam 210. The array ofbeam-limit apertures can limit the plurality of beamlets 214, 216, and218. While three beamlets 214, 216, and 218 are shown in FIG. 2A,embodiments of the present disclosure are not so limited. For example,in some embodiments, an apparatus 104 may be configured to generate afirst number of beamlets. In some embodiments, the first number ofbeamlets may be in a range from 1 to 1000. In some embodiments, thefirst number of beamlets may be in a range from 200-500. In an exemplaryembodiment, an apparatus 104 may generate 400 beamlets.

An example of source conversion unit 212 is shown in FIG. 2B. Sourceconversion unit 212 may comprise one or more micro electro-mechanicalsystem (MEMS) structures. For example, source conversion unit 212 maycomprise an optional pre-bending deflector array (PBDA) module 310, oneor more spacer substrates 320, and a micro deflector compensator array(MDCA) module 330. A primary charged particle beam may be input tosource conversion unit 212 and a plurality of beamlets may be output.

Structures included in PBDA module 310 may comprise a plurality ofsubstrates including apertures, deflectors, and electrodes, among otherstructures. The substrates may be arranged in a stacked arrangement.Similarly, structures included in MDCA module 330 may comprise aplurality of substrates including apertures, deflectors, electromagneticlenses, and electrodes, among other structures.

Condenser lens 206 can focus primary electron beam 210. The electriccurrents of beamlets 214, 216, and 218 downstream of source conversionunit 212 can be varied by adjusting the focusing power of condenser lens206 or by changing the radial sizes of the corresponding beam-limitapertures within the array of beam-limit apertures. Objective lens 228can focus beamlets 214, 216, and 218 onto a wafer 230 for imaging, andcan form a plurality of probe spots 270, 272, and 274 on surface ofwafer 230.

Beam separator 222 can be a beam separator of Wien filter typegenerating an electrostatic dipole field and a magnetic dipole field. Insome embodiments, if they are applied, the force exerted byelectrostatic dipole field on an electron of beamlets 214, 216, and 218can be equal in magnitude and opposite in direction to the force exertedon the electron by magnetic dipole field. Beamlets 214, 216, and 218 cantherefore pass straight through beam separator 222 with zero deflectionangle. However, the total dispersion of beamlets 214, 216, and 218generated by beam separator 222 can also be non-zero. Beam separator 222can separate secondary electron beams 236, 238, and 240 from beamlets214, 216, and 218 and direct secondary electron beams 236, 238, and 240towards secondary optical system 242.

Deflection scanning unit 226 can deflect beamlets 214, 216, and 218 toscan probe spots 270, 272, and 274 over a surface area of wafer 230. Inresponse to incidence of beamlets 214, 216, and 218 at probe spots 270,272, and 274, secondary electron beams 236, 238, and 240 may be emittedfrom wafer 230. Secondary electron beams 236, 238, and 240 can compriseelectrons with a distribution of energies including secondary electrons(energies≤50 eV) and backscattered electrons (energies between 50 eV andlanding energies of beamlets 214, 216, and 218). Secondary opticalsystem 242 can focus secondary electron beams 236, 238, and 240 ontodetection sub-regions 246, 248, and 250 of electron detection device244. Detection sub-regions 246, 248, and 250 may be configured to detectcorresponding secondary electron beams 236, 238, and 240 and generatecorresponding signals used to reconstruct an image of surface area ofwafer 230.

Next, an example of a single-beam electron beam tool will be discussed.As shown in FIG. 3, an electron beam tool 104A includes a wafer holder136 supported by motorized stage 134 that can hold wafer 230 to beinspected. Electron beam tool 104A includes an electron emitter, whichmay comprise a cathode 103, an anode 120, and a gun aperture 122.Electron beam tool 104A further includes a beam limit aperture 125, acondenser lens 126, a column aperture 135, an objective lens assembly132, and an electron detector 144. Objective lens assembly 132, in someembodiments, is a modified SORIL lens, which includes a pole piece 132a, a control electrode 132 b, a deflector 132 c, and an exciting coil132 d. In a general imaging process, the electron beam 161 emanatingfrom the tip of the cathode 103 is accelerated by anode 120 voltage,passes through gun aperture 122, beam limit aperture 125, condenser lens126, and is focused into a probe spot by the modified SORIL lens andthen impinges onto the surface of wafer 230. Backscattered or secondaryelectrons emanated from the wafer surface are collected by detector 144to form an image of the interest area.

A detector may be placed along an optical axis 260 of apparatus 104A, asin the embodiment shown in FIG. 3. In some embodiments, a detector maybe arranged off axis.

In some embodiments, other types of microscopy systems can be employed.For example, a transmission electron microscope (TEM) or a scanningtransmission electron microscope (STEM), etc., can be similarly applied.In a TEM system, an electron beam may irradiate a sample with generallya higher energy than in the case of SEM, so as to allow electrons topenetrate the full depth of the sample. In TEM, a diffraction patternmay be acquired from irradiating a region of interest with a finelyfocused electron probe spot. FIG. 4 shows an example of a TEM system.For example, an electron beam 460 is directed to a sample 410. Electronbeam 460 interacts with sample 410, and as a result, a plurality ofbeams 471, 472, 473 may be generated. One or more of beams 471, 472, 473may be projected onto a detector 440.

Reference will now be made to an exemplary modulation technique.

In an exemplary comparative detection scheme, as shown in FIG. 5, acharged particle source 510 emits a beam. The beam passes through anaperture 520. Aperture 520 may comprise a plurality of aperture holes.Thereafter, a plurality of beamlets are generated that travel to asample 530. After impinging on sample 530, a plurality of secondaryparticle beams are directed to a detector 540 comprising a plurality ofsensing elements. Signals received from a number n of the plurality ofsensing elements may comprise n×50 MHz signals.

FIG. 6 depicts an exemplary surface of detector 540. Detector 540comprises a plurality of individual sensing elements 601 arranged in atwo-dimensional array. Sensing elements 601 may be electron sensingelements that comprise, for example, a PIN diode, avalanche diode,electron multiplier tube (EMT), etc., and combinations thereof. Sensingelements 601 can generate a current signal commensurate with the amountof charged particles received on a surface thereof.

As shown in FIG. 7, a detector array comprising a plurality of sensingelements can detect a plurality of beam spots 701, 702, 703, and 704.Furthermore, some of the individual sensing elements can be connectedtogether so that entire beam spots are associated with a collection ofsensing elements. Connections among the plurality of sensing elementsmay be controlled by a controller (e.g., controller 109 of FIG. 1) orone or more control circuits. The plurality of sensing elements may beconfigured to be connected through a switch matrix, for example.

In some embodiments, the number of sensing elements in a detector arraycan be reduced by employing beam modulation. Further, a detectorcomprising a single sensing element, e.g., a single detection plate canbe used.

FIG. 8 illustrates an exemplary detection scheme employing modulation.In FIG. 8, a charged particle source 510 emits a beam. The beam passesthrough an aperture 520. Thereafter, a plurality of beamlets aregenerated. A modulator 800 is also provided. Modulator 800 can receivethe plurality of beamlets and output a plurality of modulated beamlets.Modulated beamlets then travel to a sample 530. After impinging onsample 530, a plurality of modulated secondary particle beamlets aredirected to a detector 541. Detector 541 may include a first number ofsensing elements where the first number is less than the number of theplurality of beamlets. Detector 541 may also consist of one sensingelement. Because the beamlets are modulated, individual secondarybeamlets may impinge on a respective sensing element of detector 541separately. Signals received from the one or more sensing element ofdetector 541 may comprise one 50 MHz signal.

Modulator 800 can be located at various positions in a charged particlebeam system. For example, modulator 800 can be placed between aperture520 and sample 530, as shown in FIG. 8. In an arrangement, for exampleas shown in FIG. 2A, modulator 800 can be placed between sourceconversion unit 212 and primary projection optical system 220.Alternatively, modulator 800 can be placed between sample 530 anddetector 541, In an arrangement, for example as shown in FIG. 2A,modulator 800 can be placed between secondary optical system 242 andelectron detection device 244. Effects and advantages of the variousarrangements will be discussed later below.

In some embodiments, modulator 800 may be integrated together withsource conversion unit 212. Modulator 800 may be provided within sourceconversion unit 212. For example, modulator 800 may be interposedbetween substrates among the plurality of substrates of the variousmodules. Modulator 800 may be provided in substrate 320.

Modulator 800 modulates the plurality of beamlets, for example, in thefollowing manner. The plurality of beamlets can be rapidly turned on/offin a manner corresponding to a coding signal. The beamlets may bemodulated according to a CDMA system configuration. With reference toFIG. 9, where the horizontal axis represents time and the vertical axisrepresents intensity in arbitrary units (AU), a beamlet duty cycle isshown in graph (a). As an example, a beamlet has a duty cycle of 1, thatis, the beam is constantly emitting from the electron source to bescanned across the wafer for imaging. Modulator 800 may generate acoding signal that may be based on random numbers or may be optimized tomaximize the amount of charged particles reaching the detector, as shallbe described later. The coding signal may be based on a pseudorandomseries of numbers, for example, as shown in graph (b). Accordingly,modulator 800 may output a PN code for each individual beamlet. Thus,the coding signal is determinable, and each coding signal associatedwith a different beamlet should have a low degree of correlation withone another so that they can be easily distinguished. The modulatedsignal may comprise a signal that is combined by bitwise XOR (exclusiveOR) with the beamlet to be modulated, as, shown in graph (c). In thisexample, because the beamlet originally has a constant duty cycle of 1,the modulated signal corresponds to the inverse of its assigned PN code.

Modulator 800 may be formed by various structures. In some embodiments,a modulator comprises a deflector that deflects an electron beam (orbeamlet). A deflector may comprise an electrostatic deflector, amagnetic deflector, and/or an element configured for inducing beamdispersion. As shown in FIG. 10A, a deflector 1010 is arranged adjacentto an aperture 1020. Deflector 1010 may include a deflecting element1011 and a deflecting element 1012 provided so that deflecting elementsare arranged on either sides of a passing beam. As shown in FIG. 10B,deflector 1010 may also be provided so that only one deflecting element1011 is arranged on a side of a beam. Deflector 1010 may be controlledby application of a predetermined voltage and/or current. Deflector 1010may be a fast switching element that can be operated at high frequency,such as on the order of GHz. Thus, a drive signal can be applied tomodulator 800 to operate deflector 1010. Deflector 1010 may be operablebetween an “on” state where a beam 1001 passes therethrough withoutbeing deflected, and an “off” state where beam 1001 is deflected. Whendeflector 1010 is activated, beam 1001 deviates from an optical axisdirection and is directed onto a surface of aperture 1020. Therefore,beam 1001 is prevented from continuing in the optical axis direction andis effectively turned off. When deflector 1010 is deactivated, beam 1001reverts to its original path.

Other means of manipulating a beam may comprise, for example, a shutter,switch, and the like. In addition to aperture 1020, an aperture 1005 maybe provided. Aperture 1005 may permit only one beam to enter a spacebetween aperture 1005 and aperture 1020, and thus only one beam reachesa respective element of a deflector. Modulator 800 may comprise aplurality of deflectors arranged in an array. Modulator 800 may comprisea substantially planar substrate with a plurality of deflectors arrangedin a two-dimensional array.

Modulator 800 may be placed before or after sample 530. When modulator800 is placed before sample 530, a detection system can be simplified.For example, a plurality of beamlets may be modulated before impingingon sample 530. In response to interacting with sample 530, a pluralityof modulated secondary beamlets may be directed to detector 541. Themodulation may result in the secondary beamlets impinging on detector541 separately. As the secondary beamlets interact with detector 541, adetection signal commensurate with the received electrons is generated.In some embodiments, there is no overlap of signals corresponding to thedifferent beamlets because modulation causes the beamlets to impinge ondetector 541 at separate times. Thus, the need for a plurality ofseparate sensing elements can be eliminated, among other things.

In an exemplary comparative detection system for a MBI system, separatesensing elements of a detector array are activated by a correspondingelectron beamlet falling onto a corresponding activation area. In orderto avoid cross talk (the unintended activation of a neighboring sensingelement), each beamlet should be well-aligned with and well-focused onan activation area. Alignment of all beamlets with respective sensingelements may require a complex optics system. As an alternativearrangement, a fine array of many separate sensing elements may beprovided, and the array can be customized via a switching matrix orfield programmable array system to manipulate connections among sensingelements so that collections of sensing elements correspond withrespective beamlets. Such collection can also be configured tocompensate for beam movement, however, may not completely eliminatecross talk. Furthermore, in either scenario, complexities areencountered due to added components and the number of signal channels.

When modulator 800 is placed before sample 530, an optical sub-systemcan be omitted, among other things, and a detection system can besimplified. For example, FIG. 11 illustrates an electron beam tool 104Bcomprising modulator 800. Modulator 800 may be configured to receiveeach of the beamlets generated by the electron source, for example,beamlets 214, 216, and 218. Modulator 800 may output modulated beamletsthat travel to wafer 230. After interacting with wafer 230, modulatedsecondary beamlets 236, 238, 240 can directly impinge on detector 244B.Secondary optical system 242 can thus be omitted, since modulatedsecondary beamlets need not be focused and aligned with respectivesensing elements. Detector 244B may comprise a single sensing elementthat may be configured as a detector plate. Since an entire surface areaof a detector plate may constitute an active area, current signals canbe generated regardless of where a beamlet lands on the detector plate.

In some embodiments, beam separator 222, which may comprise a Wienfilter, can also be omitted. For example, beams may be directed to asurface of wafer 230 in a substantially straight linear path, and neednot be bent by a Wien filter.

Furthermore, numerous other effects can be achieved. For example, due toa simplified structure of a detector plate, further flexibility insystem design and detector placement can be achieved. The detector maybe positioned closer to wafer 230, thus enhancing electron collectionefficiency. The detector may be able to detect backscattered electronsin addition to secondary electrons. For example, electrons havingdiffering energy levels may be configured to be attracted to variouscomponents held at differing potentials in an electron beam tool. Asingle detector plate may be used to gather a greater amount ofelectrons emitted from a sample, and thus, various electron signals canbe analyzed by a detector.

When modulator 800 is placed before sample 530, optical subsystems canbe maintained, if desired. For example, secondary optical system 242need not be omitted, yet a detection system can be simplified, forexample, by relaxing a focusing control of secondary electron beams 236,238, and 240 directed onto a detector.

When modulator 800 is placed after sample 530, an existing structure ofan electron beam tool can be used, while a detector can be simplified.For example, FIG. 12 illustrates an electron beam tool 104C comprisingmodulator 800. Modulator 800 may be positioned before or after secondaryoptical system 242. Modulator 800 may be configured to receive secondarybeamlets 236, 238, and 240 having been emitted from the surface of wafer230. Modulator 800 may output modulated beamlets that travel to adetector 244C.

In some embodiments, a detector can be configured to have a number ofsensing elements greater than one. For example, FIG. 13 illustrates adetector 640 comprising four sensing elements 641, 642, 643, and 644.Each of the sensing elements may be sized such that one sensing elementcan receive one or more beam spots. As shown in FIG. 13, by way ofexample, a beam spot 681 may impinge on the surface of sensing element641. Beam spot 681 may correspond to a beamlet, such as secondarybeamlet 236. Other beam spots 682, 683, 684 may correspond to beamletsthat are modulated together with beamlet 236 in one modulation group.For example, beamlets in one modulation group may be modulated so thatwhen one beamlet is in an “on” state, all other beamlets in themodulation group are in an “off” state. Because the beamlets aremodulated, only one beam spot may land on one sensing element at a time.Since a relatively wide area is provided for a plurality of beam spots,secondary optics can be configured to align beams within a larger targetarea. In some embodiments, for example, one or more of the plurality ofbeamlets may be configured to impinge roughly at the same locationcorresponding to one sensing element. Thus, beamlets may be configuredto partially or fully spatially overlap on the detector surface.Furthermore, secondary optics need not necessarily be rigorouslycontrolled to ensure that beamlets are aligned and focused on arespective target sensing element. Furthermore, the number of outputcurrent signal channels from detector 640 can be reduced.

At one extreme, a single sensing element can be provided in a detectorso that a plurality of modulated beamlets can interact with the singlesensing element. In comparison to this, some number of sensing elementscan be provided in a detector array so that one or more modulatedbeamlets interact with respective sensing elements. A system can beoptimized based on the number of output channels and bandwidth allocatedto respective beamlets.

Interpretation of an output signal channel from a detector may compriseperforming demodulation. Demodulation may be carried out internally orexternally to the detector. Demodulation processing may occur at variouslocations based on availability of computing resources, for example.Modulator 800 may generate a plurality of PN codes corresponding to theplurality of modulated beams.

FIG. 14 is a schematic representation of modulation and demodulation.Modulation and demodulation may be performed analogous to a CDMA method.Modulator 800 may be configured to receive a plurality of chargedparticle beams 1401, 1402, 1403, and 1404. Modulator 800 may beconfigured to modulate the plurality of charged particle beams 1401,1402, 1403, 1404 and to generate a modulated charged particle beam 1411.Modulated charged particle beam 1411 may be incident on a detector 544.Detector 544 may be configured to output a detected current signal 1412based on the current generated as a result of modulated charged particlebeam 1411 impinging on detector 544. For example, detector 544 maycomprise a diode. Detected current signal 1412 may be encoded due to themodulation. Therefore, to extract signals of individual beamletscorresponding to different scanned data of a sample, a demodulator 801may perform de-multiplexing. For example, modulator 800 may generate aplurality of PN codes 1431, 1432, 1433, 1434 corresponding to theplurality of charged particle beams 1401, 1402, 1403, 1404,respectively. The PN codes may be based on a series of pseudo-randomnumbers. A code generator 805 may be configured to generate the PNcodes. Demodulator 801 may use PN codes 1431, 1432, 1433, 1434 toextract current signals 1421, 1422, 1423, and 1424 corresponding to theoriginal charged particle beams 1401, 1402, 1403, and 1404.De-multiplexing may comprise performing an XOR operation with detectedcurrent signal 1412 and each of the PN codes 1431, 1432, 1433, and 1434.Demodulator 801 may comprise circuitry assisting with the demodulationfor example, such as circuitry incorporating firmware. Alternatively,demodulator 801 may comprise one or more circuits integrated withdetector 544. For example, demodulator 801 may be provided integrallywith detector as 244B as shown in FIG. 11. Demodulator 801 may be partof a wiring layer included in detector 244B. Demodulator 801 maycomprise a transceiver configured to communicate with code generator805, a memory, and other circuitry, for example.

Demodulator 801 may comprise a receiver configured to receive signalscorresponding to a plurality of modulated charged particle beams. Forexample, a receiver of demodulator 801 may be configured to receivepin-diode current from a detector. Demodulator 801 may be configured toimplement demodulation to interpret signals received from an outputchannel of a detector that receives modulated charged particle beams.Demodulator 801 may be loaded with firmware providing a modulationand/or demodulation scheme.

Detected current signal 1412 may be a single carrier line that conveysinformation associated with a plurality of beamlets. When chargedparticle beams are modulated, information of a plurality of differentregions can be transmitted in a carrier line at once. For example,modulated charged particle beam 1411 may be a single carrier line thatcomprises a plurality of modulated beamlets representing differentregions of a sample. After a detector detects current signals from thereceived modulated charged particle beam 1411, the current signals canbe de-multiplexed to reconstruct original data representing thedifferent regions.

In the scheme of FIG. 14, modulated charged particle beam 1411 may berepresented as a single line; however, based on a construction ofmodulator 800, a charged particle beam may be emitted at separatelocations. For example, modulator 800 may be configured to emit aplurality of beams according to a coding signal so that only one beam isemitted at a time. In some embodiments, for example, a plurality ofmodulation groups are provided so that a plurality of beams may beemitted at a time, each of the beams being directed to a differentsensing element on detector 544.

With reference to FIG. 15, where the horizontal axis represents time andthe vertical axis represents intensity in arbitrary units (AU), a graph(a) shows an exemplary detector signal for a corresponding beamimpinging on a sensing element. A region S1 may represent a time wherethe sensing element is not receiving any charged particles, and thus,does not generate any current. A region S2 may represent a time wherethe sensing element is receiving charged particles on a surface thereof,and thus, a detection current signal is generated. The detection currentsignal may be a PIN diode current. A charged particle system may beconfigured to scan a beam across the surface of a sample such thatdetection current signals generated based on the charged particle beam'sinteractions with the sample may be indicative of the surface topology.In this manner, imaging of the sample surface based on the detectionsignal can be carried out.

In FIG. 15, a graph (b) shows an exemplary PN code. The PN code may begenerated by code generator 805 of modulator 800. In FIG. 15, a graph(c) shows a modulated signal. The modulated signal may comprise adetector signal that is combined by bitwise XOR with a respective PNcode. For example, graph (a) and graph (b) of FIG. 15 may be XORed toyield graph (c).

When modulator 800 is located between a sample and detector, modulationmay be carried out according to FIG. 15. For example, a charged particlesystem may be configured to scan a beam over a sample surface. Afterinteracting with a sample, a beam may be directed to modulator 800. Ifthe beam were to travel directly to the detector, the beam may impingeon a sensing element and generate a detection signal such as that ofgraph (a). However, modulator 800 may receive the beam before the beamreaches the detector, and may generate a PN code such as that of graph(b). Modulator 800 may then modulate the beam so that the beam is outputfrom modulator 800 in the manner represented by graph (c). Therefore, anactual detection signal generated by a sensing element receiving themodulated beam may correspond to graph (c).

A plurality of beams can be modulated together in one modulation groupaccording to FIG. 15. For example, a plurality of PN codes can begenerated such that their respective duty cycles do not temporallyoverlap with one another. Thus, the modulated beams directed to asensing element of a detector may be configured to impinge on thesensing element at different times. The plurality of PN codes may begrouped according to an assignment of a predetermined number of beamletsto be directed to one sensing element of the detector. Meanwhile, insome embodiments, duty cycles for beams associated with codes ofdifferent groups may overlap.

Grouping of beams among a plurality of beams may be based on variouscriteria. For example, a modulation group may be set so that beamsemitted from aperture 520 that are adjacent to one another are groupedtogether. In some embodiments, a beam emitted from one aperture hole maybe grouped with beams emitted from directly adjacent aperture holes. Inthis manner, beams in close proximity may be directed to the samesensing element on a detector.

With reference to FIG. 16, where the horizontal axis represents time andthe vertical axis represents intensity in arbitrary units (AU), a graph(a) represents an exemplary charged particle beam. The charged particlebeam may be a beamlet having a duty cycle of 1. Modulator 800 maygenerate a PN signal as shown in graph (b). Modulator 800 may thenmodulate the beam so that the beam is output from modulator 800 in themanner represented by graph (c). The modulated beam may comprise asignal that is combined by bitwise XOR with the PN code, yielding a beamas represented by graph (c).

In FIG. 16, graph (d) shows an exemplary detector signal for anunmodulated beam that would impinge on a sensing element afterinteracting with a sample. A region S1 may represent a time where thesensing element is not receiving any charged particles due to the samplesurface topology, and thus, does not generate any current. A region S2may represent a time where the sensing element is receiving chargedparticles on a surface thereof, and thus, a detection current signal isgenerated. In some embodiments, a charged particle system may beconfigured to scan a modulated beam, such as that represented by graph(c) of FIG. 16, across the surface of the sample. Due to the samplesurface topology, an actual detection signal generated by a sensingelement receiving the modulated beam may correspond to graph (e).

While the above examples show beam current signals detected by a sensingelement represented as either on or off, it is understood thatintermediate levels of signals may be generated, for example, whenvarying amounts of charged particles impinge on detector sensingelements.

While a PN code has been discussed, other modulation and demodulationtechniques may be used that involve other codes as well. For example,modulation may comprise using other codes such as those applicable totelecommunications CDMA methods, such as a Gold code, Kasami code,maximum length sequence, Walsh Hadamard sequence, and the like.Furthermore, other types of modulation schemes, such as using orthogonalcoding, can be employed.

In some embodiments, a plurality of sensing elements may be provided ina detector. A charged particle imaging system may comprise a chargedparticle source configured to generate a first number of beamlets. Anumber of the plurality of sensing elements of the detector may be asecond number. The first number may be greater than the second number.For example, the first number may be in a range between, for example, 9to 441. The second number may be in a range between, for example, 4 to400. Modulation may be applied to modulate the beamlets in groups of athird number. The detector may be configured to generate output signalsin a fourth number of channels. The third number and the fourth numbermay be equal.

In an exemplary system, a charged particle apparatus may be configuredto generate 400 beamlets. Assuming one channel having a bandwidth of 50MHz, information content to be transmitted by a detector may be 400×50MHz. A detector comprising a 5×5 array of sensing elements may beprovided. That is, 25 sensing elements are provided. The beamlets may bemodulated in groups of 16, using 16 PN codes, for example. Thus, amodulator may be configured to modulate 16 beamlets together such thatone sensing element receives one of the 16 beamlets at one time. Thatis, the modulated beamlets in one group do not temporally overlap on onesensing element. One sensing element can thus process 16 beam signals,and is able to output information from all of the 16 beam signals. Eachof the 25 sensing elements may have an output channel having a bandwidthof 800 MHz. Therefore, a 5×5 detector array (comprising 25 sensingelements) can process the 400 beamlets generated by the charged particleapparatus. In a comparative example, a detector having 400 (or more)sensing elements may be configured to transmit 400 signals correspondingto each of the 400 beamlets. The detector having 25 sensing elements cantransmit the same amount of information content as that of the detectorhaving 400 sensing elements.

In other embodiments, a system can be configured with an intermediatenumber of codes and beams. For example, a modulator can be configured togenerate 10 codes. Thus, 10 beamlets may be directed to a single sensingelement of a detector. The detector may be an array comprising aplurality of sensing elements, each configured to receive a plurality ofmodulated beamlets. In some embodiments, when modulation is employed, asimplified detector can be achieved having high bandwidth.

Furthermore, a modulator may comprise a number of deflector elementsgreater than or equal to the number of beamlets to be generated. Thus, asystem has a high degree of flexibility to accommodate variousarrangements of charged beam system imaging. Additionally, astandardized component constituting a modulator can be provided to avariety of charged particle systems. For example, a modulator having anarray of a first number of deflector elements can be provided tomulti-beam charged particle systems configured to generate a secondnumber of beamlets, where the first number is greater than the secondnumber. The modulator may be configured to generate a correspondingnumber of codes based on the number of beamlets received. The modulatormay comprise one or more sensors configured to determine a number ofbeamlets received. Thus, while some of the deflector elements may notactually receive any beams, the modulator can determine the number ofdeflector elements used out of the total number of deflector elementsand generate a corresponding number of PN codes. The codes andcorresponding modulation may be optimized to ensure advantageous dutycycles of the modulated beams. Optimization may be based on the numberof deflector elements used. For example, the PN codes may be generatedsuch that the period during which deflectors deflect beams away from therespective optical axis directions is minimized, thus allowing a greateramount of charged particles to ultimately reach the detector.

Modulation may be synchronized with beam scanning. For example, in anelectron beam tool 104, all components may share basic clocks andtiming. Thus, components in detector 244 can be synchronized withcomponents of modulator 800, such as deflector 1010 and code generator805. Therefore, data recording can be aligned with beam scanning.Furthermore, components in primary projection optical system 220, suchas deflection scanning unit 226, can be synchronized with components ofmodulator 800, such as deflector 1010.

In a charged particle beam system for imaging, a charged particle beammay be generated and scanned across a sample surface with apredetermined frequency to obtain signals that represent features on thesample. For example, in some embodiments, a charged particle system maybe configured to scan across the sample under conditions of: 100 ms scanat 100 MHz. Furthermore, a charged particle beam system may beconfigured so that there are approximately 8,000 pixels in a row, sothat scan time for a line may be, for example, approximately 100 ms.Data content transmission may be configured so that its frequency isalways lower than half the frequency of the scan rate. For example, inorder to avoid aliasing, based on the Nyquist criteria, a chargedparticle beam system may be configured with a 100 MHz scan rate andsignal bandwidth of less than 50 MHz. In some embodiments, for 8-bitADC×n channels, an output rate of 1,000 MB/s with buffer size of 125 MBcan be used. Thus, a frequency to be used for an imaging channel may bein a range of, for example, 1 to 100 MHz. In some embodiments, afrequency to be used for an imaging channel may be in a range of, forexample, 20 to 50 MHz. In some embodiments, an imaging channel may use afrequency of 33 MHz. A detector comprising one or more sensing elementsmay be configured to transmit detector current using an imaging channelfrequency.

For some charged particle beam systems, other scan frequencies largerthan 100 MHz can be used. For example, a frequency of 400 MHz may beused.

In some embodiments, a modulator comprises a switchable deflector thatis operable on the order of GHz. Meanwhile, an output channel from adetector sensing element may be on the order of MHz. Thus, switchingbetween states of the deflector of the modulator may occur at a speedseveral orders of magnitude higher than a data transmission speed of thedetector. Modulation can therefore occur at a speed several orders ofmagnitude higher than that of detector signal transmission.

Optimization of codes may also be based on frequencies of a detectoroutput.

In some embodiments, a detector array can be simplified by reducing thenumber of sensing elements. A detector array can also be simplified byreducing the number of switches in a switching matrix that controlsoutputs of detector sensing elements.

In exemplary comparative detector systems, a detector may be limited bythe amount of charged particle current it is able to detect. Forexample, a detection branch may be responsible for a reduction in theamount of charged particle current detected such that approximately onehalf of the current admitted into the column is eventually output as adetection signal. Thus, capture efficiency is ½. In some exemplaryembodiments of the present disclosure, modulation may be responsible fora reduction of some amount of current, since charged particle beams maybe deflected onto an aperture surface when a deflector is operating inthe “off” state. Some exemplary embodiments using modulation may achievecapture efficiency of greater than ½, for example.

In some embodiments, a detector may communicate with a controller thatcontrols a charged particle beam system. For example, the detector maytransmit beam current output to the controller, and the controller maycontrol various functions of the charged particle beam system inresponse. Furthermore, the controller may communicate with a modulatorto transmit codes, such as PN codes. Exemplary forms of communicationmay be through a medium such as an electrical conductor, optical fibercable, portable storage media, IR, Bluetooth, Internet, wirelessnetwork, wireless radio, or a combination thereof. The controller mayreceive a signal from the detector and may construct an image. Acontroller may also perform various post-processing functions, such asdemodulation, image subdivision, generating contours, superimposingindicators on an acquired image, and the like. The controller maycomprise a storage that is a storage medium such as a hard disk, randomaccess memory (RAM), other types of computer readable memory, and thelike. The storage may be used for saving scanned raw image data asoriginal images, and post-processed images.

While a controller, storage, and one or more circuits are discussedabove as separate units, a computer may carry out the processing of allsuch units.

A non-transitory computer readable medium may be provided that storesinstructions for a processor of controller 109 to carry out modulation,demodulation, and/or image processing. Common forms of non-transitorymedia include, for example, a floppy disk, a flexible disk, hard disk,solid state drive, magnetic tape, or any other magnetic data storagemedium, a CD-ROM, any other optical data storage medium, any physicalmedium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROMor any other flash memory, NVRAM, a cache, a register, any other memorychip or cartridge, and networked versions of the same.

It will be appreciated that the present invention is not limited to theexact construction that has been described above and illustrated in theaccompanying drawings, and that various modifications and changes can bemade without departing from the scope thereof.

To expedite the foregoing portion of the disclosure, variouscombinations of elements are described together. It is to be understoodthat aspects of the disclosure in their broadest sense are not limitedto the particular combinations previously described. Rather, embodimentsof the invention, consistent with this disclosure, and as illustrated byway of example in the figures, may include one or more of the followinglisted features, either alone or in combination with any one or more ofthe following listed features, or in combination with the previouslydescribed features.

For example, there may be provided a charged particle beam system. Thecharged particle beam system may include a charged particle sourceconfigured to generate a plurality of charged particle beams; amodulator configured to receive the plurality of charged particle beamsand generate a plurality of modulated charged particle beams; and adetector configured to receive the plurality of modulated chargedparticle beams. There may also be provided:

-   -   wherein the charged particle source is configured to generate a        first number of the plurality of charged particle beams.    -   wherein the detector comprises a second number of sensing        elements, the second number being less than the first number.    -   wherein the second number is one.    -   a code generator configured to generate a plurality of codes        corresponding to the plurality of charged particle beams.    -   wherein the modulator is configured to modulate the plurality of        charged particle beams based on the plurality of codes.    -   wherein the modulator is configured to modulate a group of the        plurality of charged particle beams, the group comprising a        third number of the plurality of charged particle beams, the        third number being less than the first number.    -   wherein the modulator is configured to emit grouped modulated        charged particle beams of the group of the plurality of charged        particle beams onto a sensing element of the sensing elements.    -   wherein the grouped modulated charged particle beams of the        group do not temporally overlap one another on the sensing        element.    -   wherein the codes are configured so that a modulated charged        particle beam of the plurality of modulated charged particle        beams does not temporally overlap with another modulated charged        particle beam of the plurality of modulated charged particle        beams on the detector.    -   a controller.    -   wherein the controller is configured to demodulate the plurality        of modulated charged particle beams.    -   one or more circuits integrated with the detector.    -   wherein the one or more circuits are configured to demodulate        the plurality of modulated charged particle beams.    -   wherein the plurality of modulated charged particle beams are        demodulated based on the plurality of codes.    -   wherein the modulator comprises one or more deflectors        configured to deflect one or more of the plurality of charged        particle beams to generate the plurality of modulated charged        particle beams.    -   wherein the plurality of charged particle beams comprise a        plurality of beamlets generated based on a primary charged        particle beam emitted from the charged particle source.

There may be provided, for example, a method for modulating a chargedparticle beam. The method may include generating a plurality ofmodulated charged particle beams based on a plurality of chargedparticle beams generated by a charged particle source; and detecting theplurality of modulated charged particle beams. There may also beprovided:

-   -   generating a first number of the plurality of charged particle        beams.    -   detecting the plurality of modulated charged particle beams by a        second number of sensing elements of a detector, the second        number being less than the first number.    -   generating a plurality of codes corresponding to the plurality        of charged particle beams.    -   modulating the plurality of charged particle beams based on the        plurality of codes.    -   grouping the plurality of charged particle beams in one or more        groups, a group of the one or more groups comprising a third        number of the plurality of charged particle beams, the third        number being less than the first number.    -   emitting grouped modulated charged particle beams of the group        of the plurality of charged particle beams onto a sensing        element of the sensing elements so that the grouped modulated        charged particles beams of the group do not temporally overlap        one another on the sensing element.    -   wherein the codes are configured so that a modulated charged        particle beam of the plurality of modulated charged particle        beams does not temporally overlap with another modulated charged        particle beam of the plurality of modulated charged particle        beams on the detector.    -   demodulating the plurality of modulated charged particle beams.    -   acquiring an image of a sample based on a demodulated signal of        the plurality of modulated charged particle beams.    -   wherein the plurality of modulated charged particle beams are        received from the sample.    -   wherein the plurality of modulated charged particle beams are        demodulated based on the plurality of codes.    -   wherein the plurality of modulated charged particle beams are        generated by switching a modulator between an on state and an        off state.

There may be provided, for example, a non-transitory computer readablemedium comprising a set of instructions that are executable by one ormore processors of an apparatus to cause the apparatus to perform acomputer-implemented method. The computer-implemented method may includedriving a modulator to generate a plurality of modulated chargedparticle beams using a plurality of modulation codes, the plurality ofmodulated charged particle beams based on a plurality of chargedparticle beams generated from a charged particle source. There may alsobe provided:

-   -   detecting the plurality of modulated charged particle beams.    -   generating a first number of the plurality of charged particle        beams.    -   detecting the plurality of modulated charged particle beams by a        second number of sensing elements of a detector, the second        number being less than the first number.    -   generating the plurality of modulation codes corresponding to        the plurality of charged particle beams.    -   modulating the plurality of charged particle beams based on the        plurality of modulation codes.    -   grouping the plurality of charged particle beams in one or more        groups, a group of the one or more groups comprising a third        number of the plurality of charged particle beams, the third        number being less than the first number.    -   emitting grouped modulated charged particle beams of the group        of the plurality of charged particle beams onto a sensing        element of the sensing elements so that the grouped modulated        charged particles beams of the group do not temporally overlap        one another on the sensing element.    -   wherein a modulated charged particle beam of the plurality of        modulated charged particle beams does not temporally overlap        with another modulated charged particle beam of the plurality of        modulated charged particle beams on the detector.    -   demodulating the plurality of modulated charged particle beams.    -   acquiring an image of a sample based on a demodulated signal of        the plurality of modulated charged particle beams.    -   wherein the plurality of modulated charged particle beams are        received from the sample.    -   wherein the plurality of modulated charged particle beams are        demodulated based on the plurality of modulation codes.    -   wherein the plurality of modulated charged particle beams are        generated by switching a modulator between an on state and an        off state.

There may be provided, for example, a modulator. The modulator mayinclude a first aperture comprising a first aperture hole; a codegenerator configured to generate a modulation code; a deflectorconfigured to direct a charged particle beam received in the modulatoronto a surface of the first aperture according to the modulation code.There may also be provided:

-   -   wherein the deflector is operable between two states.    -   wherein in one state, the charged particle beam is directed        through the first aperture hole, and in the other state, the        charged particle beam is directed onto the surface of the first        aperture.    -   wherein the code generator is configured to generate a plurality        of pseudo-random number codes as the modulation code.

There may be provided, for example, a demodulator. The demodulator mayinclude a receiver configured to receive signals corresponding to aplurality of modulated charged particle beams; and circuitry configuredto demodulate the signals of the plurality of modulated charged particlebeams based on modulation codes. There may also be provided:

-   -   wherein the plurality of modulated charged particle beams do not        temporally overlap on the detector.    -   a transceiver configured to receive the modulation codes from a        code generator.    -   wherein the demodulator is integral with a detector that        provides the signals.    -   wherein the demodulator is part of a controller of a charged        particle beam system.

What is claimed is: 1-30. (canceled)
 31. A modulator comprising: a firstaperture comprising a first aperture hole; a code generator configuredto generate a modulation code; and a deflector configured to direct acharged particle beam received in the modulator onto a surface of thefirst aperture according to the modulation code.
 32. The modulator ofclaim 31, wherein the deflector is operable between two states, whereinin one state, the charged particle beam is directed through the firstaperture hole, and in the other state, the charged particle beam isdirected onto the surface of the first aperture.
 33. The modulator ofclaim 31, wherein the code generator is configured to generate aplurality of pseudo-random number codes as the modulation code.
 34. Ademodulator comprising: a receiver configured to receive signalscorresponding to a plurality of modulated charged particle beams; andcircuitry configured to demodulate the signals of the plurality ofmodulated charged particle beams based on modulation codes.
 35. Thedemodulator of claim 34, wherein the plurality of modulated chargedparticle beams do not temporally overlap on the detector.
 36. Thedemodulator of claim 34, further comprising a transceiver configured toreceive the modulation codes from a code generator.
 37. The demodulatorof claim 34, wherein the demodulator is integral with a detector thatprovides the signals.
 38. The demodulator of claim 34, wherein thedemodulator is part of a controller of a charged particle beam system.